The Impromptu Concert series began its 41st year two Sundays ago with a performance by the Harlem String Quartet. The venue was, as usual, St. Paul's Episcopal Church at Eaton and Duval. The quartet -- Ilman Gavilan and Melissa White, violins, Jaime Amador, viola, and Matt Zalkind, cello -- played on two opposing sides of the musical tradition with music by Mozart, Schubert, Chick Correa and Billy Strayhorn. They played all of it very well.

Leading off the concert with a quartet by Mozart, the players got a nice blend with their sounds meshed. They caught Mozart's sweetness. It was not exceptional Mozart and it was played competently if not brilliantly, but it was Mozart, unquestionably so from the first notes, and as always very welcome.

Next they moved into the jazz portion of the concert, with the composition by Chick Corea (with whom they have collaborated) and with "Take the 'A' Train," by Duke Ellington's partner Billy Strayhorn. I was doubtful. Attempts in the past to combine the two traditions (and there have been many) have seldom met with much success. Paul Whiteman's showcasing of jazz greats like Bix Beiderbecke with lush banks of strings, the French pianist Jacques Loussier's playing of Bach's music to jazz rhythms, the so-called "Third Stream" music of Gunther Schuller and Ran Blake, all seem to me like novelties, however accomplished. George Gershwin is usually cited as the composer most successful at that melding: to my ear he was on his way, but not there yet, and the tragedy of his early death is in the loss of the music he'd have written once he got there.

And the fact that these musicians were playing stringed instruments made what they were trying to do even more difficult. Strings are capable of a musical finesse that no other instruments can produce, which leaves jazz behind at lower levels of refinement, but string players find it hard to swing, or to play syncopated rhythms. And jazz is played by trumpets and saxophones, instruments far more aggressive than strings, and it is driven from behind by a drum rather than being directed from in front by a stick, all of which gives it an energy that classical music can rarely match.

Still, times have changed. The 1950s brought on a revolution in rhythm, a cultural development so powerful and deep-rooted and so badly needed that it even made a star out of the likes of Elvis Presley. And since then, musicians of every stripe have grown up with strong rhythms, the kind that are always in the background, even in music in elevators and supermarkets. The success of the resultant linking of the two traditions can be seen in the playing of Wynton Marsalis, who was twice awarded Grammys for both classical and jazz recordings in the same year.

As soon as the Harlem group at the Impromptu concert started in on "A Train," doubt went away. The first thing one heard was that they were genuinely in the jazz tradition: they did not sound like a group of classical musicians trying to play jazz. Their solos were accurate to the chord changes and all sounded interesting. The best part of it was the cellist playing the bass part, plucked to the rhythm of the song, holding everything together and pushing the time just enough to give the piece momentum. It was a nice bit of work.

Still and all, the high point of the concert was on the classical side, in the group's playing of Schubert's Quartet in D Minor, the one known as "Death and the Maiden." This is one of the great quartets and the musicians did it justice. Throughout, their sound filled the church. Their playing was, rhythmically, absolutely secure and in the faster passages it had a powerful, driving quality. In the second, Andante, movement, the dynamics, from soft to loud and back again, were perfectly in control. In the Presto fourth movement, with some technically difficult passages, the playing was passionate and there were no compromises: these people commit themselves completely to their instruments, holding nothing back. They didn't need a drum.